Conventionally, in various products such as paint, ink, fiber treatment agents, rubber, plastic molded products, synthetic leather and fiber, improvement of various properties has been conducted by adding a specific modifying material.
In order to add such modifying material to a product, a material prepared by pulverizing starting material of a modifying material to form modifying powder is used.
As such modifying powder, silk powder (powdery silk) such as powder of silk fibroin (silk fibroin powder) is utilized. By adding such silk powder, it is possible to provide a product with various properties such as silk-specific feeling, ink-accepted property and moisture absorbability/releasability. As for such products, for example, silk fibroin powder-containing artificial leather, which is produced by incorporating silk fibroin powder in synthetic resin, and the like are known.
Various production methods have been proposed for obtaining such silk powder. However, for example, when a composition prepared by compounding a silk powder to synthetic resin, or an aqueous or solvent-based liquid medium constituting ink or paint is made into a commercial product for industrial applications, control of the powder properties is required because dispersibility into thermosetting resin or dispersibility into a solvent-based or aqueous liquid medium is determined to be important. In order to control such properties, a method for obtaining powder of silk fibroin has been employed in which silk fibroin to be a silk starting material is subjected to a chemical treatment, heating/pressurizing treatment or the like. However, there are such problems that dispersibility of the obtained powder of silk fibroin is insufficient, and that the silk-specific feeling and the like can not be maintained. Accordingly, in these years, studies have been proceeded while giving attention to control the average particle diameter of silk powder to be obtained through a pulverization treatment such as a mill.
For obtaining powder of silk such as silk fibroin, using a ball mill or a jet mill is known.
However, when silk fibroin is pulverized with a ball mill, since it is necessary to change a ball mill sequentially from that with a large diameter to that with a small diameter in order to produce powder having an average particle diameter of 10 μm or less, there are such problems that pulverization time becomes very long and, in addition, the powder may color.
Also, when silk fibroin is pulverized with a jet mill, since shape of the pulverized product becomes fibrous, there is such a problem that compounding of the product to a solvent-based resin solution or an aqueous resin solution leads to increase in viscosity thereof; thereby resulting in difficulty of compounding. As described above, since powder of silk fibroin obtained by such single mechanical pulverization includes particles with irregular sizes, there is such a problem that powder of silk fibroin having a uniform size can not be obtained.
On the contrary, as a method for obtaining powder of silk fibroin suitable for a modifying material, there is provided a method for obtaining silk fibroin fine powder having an average particle diameter of 10 μm or less, for example, by pulverizing silk fibroin with a multistage pulverization method (for example, Patent Document 1). According to the production method, silk fibroin fine powder is obtained by employing a ball mill as a second stage pulverizer while determining the pulverization treatment time to be 20 hours or less, by employing a jet mill as a third stage pulverizer, and by conducting β-modification treatment during or after the pulverization treatment.
On the other hand, as modifying powder, polysaccharide powder (powdery polysaccharide) represented by cellulose powder originating from pulp is utilized. Addition of such polysaccharide powder gives various products in which various properties such as ink-accepted property and moisture absorbability/releasability are created or improved, and the product is used in various fields such as cosmetic, healthy foods and a filler for various synthetic resin products.
Various production methods have been proposed for obtaining such polysaccharide powder. However, for example, when a composition prepared by compounding a silk powder to synthetic resin, or an aqueous or solvent-based liquid medium constituting ink or paint is made into a commercial product for industrial applications, control of the powder properties is required because dispersibility into thermosetting resin or dispersibility into a solvent-based or aqueous liquid medium is determined to be important. Accordingly, studies have been proceeded while giving attention, in particular, to control the average particle diameter of polysaccharide powder.
For obtaining powder of polysaccharide such as cellulose, usage of a ball mill or a jet mill is known. However, also in such pulverization of cellulose and the like, there are problems similar to those in pulverization for obtaining silk powder described above.
On the contrary, for obtaining polysaccharide powder suitable for a modifying material, a method for producing polysaccharide powder is provided in which, for example, a first pulverization process is conducted to pulverize polysaccharide or the like using a rolling ball mill or the like within 20 hours, and then a second pulverization process is conducted to pulverize the pulverized product in the first process using a jet mill (for example, Patent Document 2).
[Patent Document 1] Japanese Patent Application Laid-Open No. 6-339924 ([Claim 1], [0012])
[Patent Document 2] Japanese Patent Application Laid-open No. 9-99251 ([Claim 1], [0023])